(1) Field of the Invention
The present invention relates to a free-ink type writing instrument having an ink collector which temporarily stores ink flowing out when the pressure inside the ink tank increases.
(2) Description of the Prior Art
There have been free-ink type writing instruments which include a temporary ink collector arranged radially outside, and separated from, the ink feed (or pen core) connected to the ink tank and made up of a molding having a comb-toothed section or which include a temporary ink collector made up of a porous structure connected to the ink feed. In such free-ink type writing instruments, it is usual that a water-based ink having a surface tension of 30 mN/m or greater at a temperature of 25xc2x0 C. is charged in its ink tank.
In such a free-ink type writing instrument, ink is pushed out from the ink tank when the pressure inside the air volume in the ink tank increases, and the ink is temporarily held in the ink collector, which prevents ejection of ink from the pen tip.
In recent years, traveling by airplane has become commonplace. When a free-ink type writing instrument is carried under such circumstances, the differential pressure between the interior of the ink tank and external atmosphere changes sharply. Therefore, the air volume inside the ink tank expands and this may cause ink leakage from the pen tip. For example, there is a free-ink type writing instrument which will not cause any ink ejection or leakage when the temperature changes are within the range of 25xc2x0 C. to 50xc2x0 C. under a normal environment. Even with such a pen, if it undergoes a more severe pressure change than a temperature change from 25xc2x0 C. to 50xc2x0 C. due to in-flight pressure in a plane in transit dropping to about 0.8 to 0.7 atm., the free-ink type writing instrument of the conventional design will suffer from ink leakage from the writing tip.
Conventionally, oil-based makers have been used as writing instruments for drawing on a non-absorptive writing surface having poor wettability, such as of polypropylene or the like because their drawn lines dry fast and no drawn line is repelled. Generally,such makers use organic solvents having a vapor pressure higher than water. An ink having a high vapor pressure sharply changes in vapor pressure with change in temperature, so the pressure of the empty space inside the ink tank increases shapely. As a result, when the air expands so that the pressure is relieved, the problems of ink leaking from the writing point often occur.
For example, the water vapor pressure of a water-based ink is about 23.8 mmHg at a temperature of 25xc2x0 C. If lines are drawn on non-absorptive surfaces such as plastic films of polypropylene, polyethylene, polyethylene terephthalate, etc., glass surfaces, metallic surfaces and the like, the drawn lines cannot but exhibit poor dryability. There is also another problem of drawn lines being repellent when a water-based ink having a surface tension of about 30 mN/m or greater at a temperature of 25xc2x0 C. is used to draw lines on a writing surface with poor wettability, e.g., resin surfaces of polypropylene, polyethylene, etc., or glass surfaces and the like with sebum adhering thereon.
It is therefore an object of the present invention to provide a free-ink type writing instrument which is able to use an ink having resistance against drawn line repellence on a writing surface with a poor wettability and excellent in drawn line dryability, still can retain all the ink flowing out from the ink tank by the temporary ink collector without causing any ink leakage such as ink ejection from the writing tip if a sharp change in external pressure or sharp expansion or contraction of air inside the ink tank occurs and which can be assembled easily at a low cost by automation.
In order to achieve the above object, the present invention is configured as follows:
In accordance with the aspect of the present invention, a free-ink type writing instrument includes: an ink tank for holding ink; a temporary ink collector for temporarily retaining the ink flowing out from the ink tank; a partitioning wall separating the ink tank and the temporary ink collector; and a pen core disposed from the ink tank, penetrating through the partitioning wall and the ink collector so as to allow ink to flow out from the ink tank, and is characterized in that the ink stored in the ink tank has a vapor pressure ranging from 20 mmHg to 60 mmHg at a temperature of 25xc2x0 C. and a surface tension of less than 30 mN/m at a temperature of 25xc2x0 C., and the storage volume Vc of the ink collector and the ink tank volume Vo satisfy the relation Vcxe2x89xa70.18 Vo.
In the present invention, any of the following features is effective: that the main solvent is of an organic solvent or water; that the temporary ink collector is a sectionally comb-shaped, columnar molding formed of a multiple number of plates arranged in layers predetermined gaps apart or a group of multiple vanes arranged in layers with gaps so as to be able to retain ink within the gaps; and that the pen core is of a fiber bundle core.